Many processes and operations in current use result in fluid waste products which must be treated before they can be recycled for re-use in the process or operation from which they have been derived, or to meet ever increasingly severe requirements before they can be discharged into the environment. Sources of such waste materials include the chemical, pharmaceutical, food processing and brewing industries; agricultural sources, such as animal husbandry, notably intense rearing of poultry, cattle and pigs, and abattoirs; and municipal and other sewage systems. Many of such waste materials are water based and it is desirable to reclaim that water for re-use and/or to purify it to sufficient extent to be able to recycle the treated water into the water usage system, for example by discharge of the treated water into rivers, lakes or reservoirs.
The term fluid waste material will be used hereinafter to denote in general a waste material from any source which has a fluid, notably water, as a major component. For convenience, the invention will be described hereinafter in terms of the treatment of municipal sewage, that is the discharge from foul water drainage systems, as the fluid waste material.
A typical method for the treatment of such waste material is to subject the material to aerobic boilogical digestion in the fluid phase so as to reduce the biological oxygen demand of the treated water to within mandatory limits so that it may be discharged to the environment. Such treatment can take place in the fluid phase in an aerated or agitated vessel containing the waste to be treated. A preferred form of such a fluid phase digestion treatment is known as the activated sludge treatment. In such a process, aeration keeps the solid particles of waste suspended in the fluid phase and enables bacteria, fungi, protozoa, metazoa and other boilogical organisms to utilise the suspended and soluble components of the sewage for the digestion process during which a biomass is built up. For convenience, the invention will be described hereinafter in terms of such an activated sludge process.
During an activated sludge process, a biomass is produced as a result of the bacterial digestion of components within the fluid waste. The term biomass is used herein to denote the live and dead bacteria and other organic materials dissolved in or suspended in the fluid phase of the digestion mixture. Part of this biomass accumulates as flocs or particles which are suspended in the aqueous phase and settle out in a subsequent settling tank or other sedimentation operation. The biomass and settled solids must be removed from the system to prevent their accumulation to excessive extents. It is also required by legislation that the effluent water contain less than stated levels of various materials and suspended solids before it can be discharged to the environment. The removed biomass and other solids are disposed of as a waste product from the system, although part of the biomass may be recycled for admixture with incoming sewage to provide and maintain the necessary bacterial population for the digestion stage. The treated water can then be discharged as the effluent from the system or may be subjected to further treatments.
The regulations and legislation governing the nature and quality of the effluent and the disposal of the biomass are becoming increasingly stringent. These, and awareness of the environmental effects of disposal of the biomass, are making conventional disposal techniques unacceptable. For example, the use of the biomass, or sewage sludge as it is also known, as a fertiliser on the land, or disposal in the sea are no longer acceptable. The cost of disposal of the biomass now represents about 60% of the cost of operating an activated sludge process and the need for an economic yet environmentally acceptable method for reducing the problem of biomass disposal still exists.
One proposal for reducing the problem has been to reduce the amount of biomass produced by the activated sludge process by the use of predator organisms which graze upon the bacteria causing digestion of the waste material. Ciliated protozoa and other metazoa, such as rotifers and oligochaete worms, are stated to be predators for the types of bacteria normally present in an activated sludge process. However, they are normally present in only a few tens or hundreds of organisms per litre in a conventional activated sludge process and do not normally have any significant effect on the digestion process and the production of biomass. It has been proposed that such predators could be held on a filter trap so that they are not swept through the system by the volumes of water being treated and can accumulate and thus have an effect on reducing the amount of biomass produced. However, this was a laboratory proposal using synthetically cultivated bacteria and ciliate predators using a synthetic nutrient composition. In a practical process, the population of such predator organisms which could be maintained is restricted by the available nutrient level in the waste material. As stated above, in a conventional activated sludge process, this population is too small to have any significant effect on the amount of biomass produced.
We have now found that if the median primary particle size of the biomass and other solids in the digestion stage of an aqueous waste material aerobic bacterial treatment process, notably an activated sludge process for the treatment of municipal sewage, is reduced to less than 10, preferably from 1 to 6, notably from 1 to 3, micrometres, then the quality, notably the suspended biomass solids content, of the treated water is enhanced. We have found that this particle size range corresponds to that at which ingestion of solid particles by rotifers and other predator organisms, such as ciliate filter feeders, is optimised and that the organisms obtain the nutrients from the ingested solids without the need to rupture the cell walls. As a result, the ingestion of biomass and other solids by the predator organisms in the waste water is increased as compared to where the biomass particle size range contains significant proportions of particles outside this range. The optimisation of the particle size for ingestion by the predator organisms enables a higher population of the predator organisms in the waste digestion stage to be supported than where no comminution has taken place. As a result, a useful reduction in biomass production in the digestion stage can be achieved without the need for additional nutrients or the expenditure of uneconomically large amounts of energy.